Add unchop to array and unchop, repeat_str and mul to string

[dipinhora]

This commit adds the following:

• Update chop in string and array to allow is to work correctly
  after an unchop
• unchop to both array and string to allow the un-chopping of a
  chopped string or array
• repeat_str and mul to string to allow easy repeating of
  strings

[dipinhora]

I originally needed the unchop function, but then I wanted an easy way to repeat strings as part of writing tests for unchop. This PR has been created as a draft as the unchop name isn't exactly great.

Let me know if the preference is that some or all of these changes go through the RFC process instead.

[jemc]

Seems reasonable to me. And useful - I can think of some things I've worked on in the past where I'd have liked this to exist.

Does it also make sense to have an unchop_val as well?

[dipinhora]

Not sure about unchop_val. The use case I had in mind for unchop required needing to get the original mutable array or string back to what it was before it was chopped. For val strings and arrays, you can keep a reference to the original if needed when you trim since both the original and the trimmed ones are immutable.

[jemc]

I was thinking about use cases where you chop a buffer to get two iso buffers, send them to separate actors to mutate them, then convert them to val so they can be shared, and then you want to eventually put the pieces back together into a single val.

However, I suppose it can wait until someone is writing a real-world program that actually has this need.

[dipinhora]

I would accomplish the same thing by assembling them back together before converting to val to share.

However, if you think it would be useful and regularly needed, I can add it in. Let me know.

[jemc]

We can wait until the need arises in a real application.

[rkallos]

From the looks of it, this docstring appears to be incorrect, as mul does not accept an argument for a separator.

[dipinhora]

Good catch. Fixed.

[SeanTAllen]

Is this ready for final review @dipinhora ?

[dipinhora]

@SeanTAllen yes, marking as ready for review now.

[SeanTAllen]

@dipinhora can you add release notes to this PR to be included in the "happening very soon" release?

[dipinhora]

release notes (possibly):

Adds unchop to both array and string to allow the the original string or array to be recovered by combining the two pieces from a previous chop.
Adds repeat_str and mul to string to allow easy repeating of strings (such as 'abc' * 6 to get abcabcabcabcabcabc or 'abc'.repeat_str(6, ', ') to get abc, abc, abc, abc, abc, abc)